The present invention relates to a method and an apparatus for charging a rechargeable battery. More particularly, the present invention is directed to voltage controlled charging of batteries connected to a protection circuit.
NiCd (Nickel-Cadmium) and NiMH (Nickel-Metal-Hydride) rechargeable batteries are conventionally charged with a relatively high constant charging current until they provide an indication of full charge. Such indications include a sudden increase in the battery temperature and a drop in the terminal voltage of the battery. However, batteries based on lead and lithium chemistries (including lithium-ion, lithium-polymer and lithium solid state) do not provide these types of indications when reaching a fully charged state. Consequently, a conventional process for charging a battery with this type of chemistry involves monitoring the battery voltage to determine when the battery is fully charged.
A conventional lithium-based battery pack has maximum voltage specifications that should be observed for safe charging. One of these is a specified maximum charging voltage that is allowed to be applied to the terminals of the battery pack. It is desirable to keep the charging voltage at or below this predetermined maximum to avoid excessive generation of heat from resistive losses in the terminals.
A conventional electrical model for a lithium-based battery pack includes terminals and a battery electrically separated from the terminals by a terminal resistance and a protection diode. When charging current is applied to the battery pack, a voltage drop develops between the battery pack terminals and the battery inside it. The battery itself may be viewed as a series electrical connection between an ideal battery (comprising one or more cells) and an internal resistance. When charging current is applied to the battery pack, a voltage drop also develops between the internal resistance and the ideal battery.
A terminal voltage is measured across the terminals of the battery pack. The terminal voltage is the sum of two voltages: an internal battery voltage developed across the ideal battery, also known as an electrode voltage, and a terminal voltage drop from the battery pack terminals to the ideal battery. To determine the electrode voltage, the terminal voltage drop may be subtracted from the terminal voltage based on prior knowledge of the magnitude of the terminal voltage drop. Alternatively, the electrode voltage may be determined by measuring the battery voltage with negligible charging current.
Another important voltage specification of a lithium-based battery pack is an end-of-charge voltage. This is the electrode voltage developed across the ideal battery when it is fully charged. It is desirable to keep the electrode voltage from exceeding the end-of-charge voltage to avoid damage to the battery inside the battery pack.
The battery pack commonly includes a protection circuit that monitors a battery voltage, which is a voltage across the battery inside the battery pack. When charging with a high charging current, the monitored battery voltage is larger than the electrode voltage due to an internal voltage drop developed across the internal resistance of the battery. When charging with a low charging current, the internal voltage drop is small and the monitored battery voltage is close to the electrode voltage.
The protection circuit is conventionally used as a safety device which may be triggered to avoid substantially overcharge of the battery. The threshold voltage of the protection circuit may be set so that the protection circuit triggers when the monitored battery voltage exceeds the end-of-charge voltage by a predetermined amount.
The electrode voltage of the battery is kept at or below the end-of-charge voltage to prevent overcharging. In a conventional charging method, the maximum charging voltage may be defined as the end-of-charge voltage. The provided battery terminal voltage should not exceed this end-of-charge voltage.
Such a conventional method of charging a lithium-based battery avoids overcharging. However, the terminal voltage reaches the end-of-charge voltage very quickly when compared to the electrode voltage. This results in a decrease in the charging current at an early stage and a consequent inefficiency in charging.